[REVIEW-3-5][REVIEW-3-6] Fix for fdo#50990 - Visio document imported as empty page

[Fridrich Strba]

Hello, good people,

During a part of the hackweek, I worked on refactoring the libvisio VSD
parser to be able to handle documents that are generated by Visio SDK,
most probably the document attached to fdo#50990 was generated like that.

The commit in master is
http://cgit.freedesktop.org/libreoffice/core/commit/?id=46f61ff87098dad62e32aa162c4ccfa71850550a

The refactoring passed our regression tests and I tested it with some of
the documents attached to fixed bugs in bugzilla that concerned Visio.
It would be good to have it in both 3.5.6 and 3.6.1 at least.

Cheers

Fridrich